Pneumatic retards are conventionally utilized in various types of alarm systems, to assure that the condition to be detected continues for a sufficient period to assure its existence. For example, the detection of water flow in the pipes of alarm systems, such as would indicate the discharge of a sprinkler head, might be falsely signalled by a single surge of water in the pipe; hence water flow should continue for a chosen number of seconds before an alarm is signalled.
Pneumatic retards of a type now used operate by releasing a plunger which, under spring pressure, slowly moves over a predetermined stroke and forces the air out of a pneumatic chamber through a constricted passage. At the end of its stroke it actuates one or more signal switches. There are numerous patents on such constructions, some of which recognize the need for a rapid, snap-action to actuate the switches. For example U.S. Pat. No. 2,627,919 shows how an electric switch may be actuated by a plunger of a pneumatic retard, the switch being given a snap action by mechanism associated with a switch itself. Applicant is unaware of any prior mechanism in which a rapid actuating movement is imparted near the close of the plunger stroke by suddenly increasing the speed of the plunger itself.
Alarm systems of the type described conventionally include two switches, one of which may actuate a local alarm and the other transmit an alarm signal to a remote alarm center. A simple but undesirable way of doing this is to have both switches physically actuated by the slow movement of the plunger. Both signals should be sent simultaneously; difficulties would be caused if the condition detected continued long enough to actuate one of such alarms but not the other. This may happen if two switches are to be operated by a slowly moving retard plunger; mechanical tolerances of electrical switches physically ganged may cause one to lag after the other is actuated.